Validity of CO2-rebreathing cardiac output during rest and exercise in young adults

To validate the CO2-rebreathing method ( Defare 's method) for estimating cardiac output in children and young adults, measurements were compared to thermodilution ( TDCO ) cardiac output in 16 subjects (age 7-19 yr) with congenital heart disease. Data were collected at rest (N = 11) and during 4-min stages of supine bicycle exercise (N = 13). Estimated arterial-venous (-v-a)CO2 content differences related linearly to the measured CO2 content difference (Y = 0. 29X + 2.47, r = 0.65, P less than 0.001). With this (v-a)CO2 difference correction for all patients (N = 16), the correlation between CO2-rebreathing cardiac output and the TDCO was r = 0.87 (SEE = +/- 1.8 l X min-1). The correlation was higher for exercise (r = 0.81) than for rest (r = 0.65). We conclude that the CO2-rebreathing method, with a (v-a)CO2 content difference adjustment, is a simple, noninvasive technique providing estimates of cardiac output in children and young adults with congenital heart disease. Individual estimates should be treated with caution, especially when used for clinical evaluations.